BWRs have conventionally utilized active safety systems to control and mitigate accident events. Those events varied from small break to design base accidents. Passive safety systems have been studied for use in BWRs because of their merits in reducing maintenance and surveillance testing of the safety-related equipment, and in eliminating the need for AC power, thereby improving the reliability of BWR operation and safety. Simplified BWRs (SBWRs) have been designed with passive safety features that provide more resistance to human error in accident control and mitigation.
In accordance with the conventional SBWR containment design, a passive containment cooling system (PCCS) eventually purges essentially all of the drywell nitrogen to the wetwell. This reduction in noncondensable concentration permits the PCCS to operate at full heat removal efficiency. This rate of heat removal will eventually exceed the rate of decay heat production. The drywell pressure will decrease to a value slightly below the wetwell pressure, resulting in opening of the wetwell to drywell vacuum breakers (WWVBs). This will result in a flow of water vapor and nitrogen back into the drywell. Since these WWVBs are designed for protection against large, suddenly imposed negative pressure differentials between wetwell and drywell, the flow rates when the WWVBs open are quite large.
In the passive safety system of the SBWR, the location and size of the WWVBs are such that more nitrogen is admitted to the drywell than is needed to degrade the PCCS heat removal rate to just balance the decreasing decay heat without overshoot. A resulting increase in drywell pressure occurs because of the temporary excess nitrogen inventory in the drywell which reduces the heat transfer efficiency of the PCCS. This excess of nitrogen will require a drywell pressure increase to clear the PCCS vent for control of non-condensables and purge the excess nitrogen back into the wetwell along with some steam. This transient can in turn overshoot and result in another opening of the WWVBs.
The above-described behavior has two negative aspects. First, repeated clearing of the PCCS vent may carry additional steam to the suppression pool, resulting in additional heating of the upper portion of the pool. Second, during the periods of a positive drywell-to-wetwell pressure difference, steam leakage to the wetwell could occur, causing a further increase in wetwell pressure. The resulting energy addition to the suppression pool and wetwell causes an undesirable gradual increase in containment pressure.